This invention relates generally to a monitor for providing an indication of the relative velocity between a moving body or surface and a fixed body or surface. The disclosure will be facilitated by particular reference to monitoring the speed of a continuous moving member such as a belt.
In many types of machines, apparatus and systems, it is necessary or desirable to monitor the speed of a shaft, a belt, or the like, and to provide indications when the speed thereof varies from a preselected or desired speed, or to provide a continuous indication of the proportion that the speed thereof bears to a predetermined desired speed. Further, in some machines it is desirable to provide means for shutting down the machine when the speed of the moving member thereof being monitored falls below a certain preselected speed, indicating a possible malfunction of the machine.
For example, in a grain storage elevator, a relatively long continuous belt with buckets or the like attached thereto is used to carry grain to a loading port or the like, commonly located at the top of the storage bin. Such belts are often as much as 300 to 400 feet in length. The continuous belt is generally driven by an electric motor. Thus, it is desirable to provide a monitor for the continuous belt, to insure proper operation thereof. Monitoring the speed of a shaft or the like driving the belt is generally not satisfactory, as the belt may slip somewhat with respect to the shaft or other drive means. Moreover, it is desirable to monitor the speed of the belt directly, for comparison with the speed of the drive means, thereby determining the amount of slippage, if any, of the belt. In the case of monitoring the movement of the belt of a grain elevator, it is important to give a warning or other indication to an operator thereof in the event the belt jams or slips in a manner which might cause over-heating and/or a fire in the belt itself or in the drive means or motor. Clearly, it is important to minimize the possibilities of fire due to equipment failure in grain elevators. It is also known to provide a magnetic pick-up coil, actuated by the bucket bolts in the belt. However, this approach is unsatisfactory in the case where non-ferrous bolts are utilized, or the buckets are secured to the belt by other means than discrete, regularly spaced, ferrous fasteners. Also, results obtained by this method are subject to innaccuracies due to such factors as misaligned or missing buckets or bolts, irregularly indented bolts, imperfections or overlapping splices in the belt, and the like.